Liquid crystal composition

ABSTRACT

A nematic liquid crystal composition having a notable increase of the twistability (i.e. the reciprocal of the intrinsic helical pitch) with temperature rise is provided, which composition comprises (i) optically active substance(s) which are the same in the helical twist sense to one another and make positive the temperature dependency of the twistability of the cholesteric phase induced when singly added to nematic compound(s), and (ii) nematic liquid crystal(s), which optically active substance(s) are a specificed compound (I) containing a plurality of benzene, cyclohexane, dioxane, pyrimidine or/and pyridine ring and also having an optically active terminal group ##STR1## (R 1  : alkyl); and further a nematic composition having a nearly constant twistability with temperature rise is provided, which composition comprises the above compound (I), optically active substance(s) which have the same helical twist sense as that of (I) and make negative the temperature dependency of the twistability of the induced cholesteric phase when singly added to nematic compound(s) and nematic liquid crystal(s), which latter optically active substance(s) are a specified compound (II) containing a plurality of benzene, cyclohexane dioxane, pyrimidine or/and pyridine group and having an optically active terminal group ##STR2## (W: a specified group, S: 0-4, and R 5  : alkyl).

This is a division of application Ser. No. 06/890,653, filed Jul. 30,1986, now U.S. Pat. No. 4,780,240.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a nematic liquid crystal composition. Moreparticularly, it relates to a nematic liquid crystal composition which,when used for liquid crystal display elements, improves the temperaturedependency of the threshold voltage thereof and also improves thetemperature dependency of the intrinsic helical pitch (hereinafterreferred to as "P").

2. Description of the Prior Art

As to TN type liquid crystal display elements, their use applicationshave been rapidly enlarged due to improvements in circuit, driving modeand cell preparation technique and particularly improvement in thecharacteristics of liquid crystal compositions sealed in the elements,although their use applications has been directed only to watches,electric calculators, etc. in theirs initial use.

Such a rapid enlargement of the use applications due to improvements inliquid crystal compositions and others owes a great deal to 1 increasein the display capacity and 2 broadening of the temperature range of thenematic liquid crystal phase.

With respect to the increase in the display capacity, displays ofhand-held computors and liquid crystal television are illustrated as useapplications. With respect to the broadening of the temperature range ofnematic liquid crystal phase, displays for instruments used on cars andinstruments for outdoor use are illustrated. However, there are a largenumber of matters to be improved for liquid crystal display elements.Examples of such matters are narrow angle of view, inferior contrast,low response speed, still yet small display capacity, reduction in thedisplay quality due to ambient temperature change, etc. Among these, thereduction in the display quality due to ambient temperature change isattributed to the change in the threshold voltage V_(th) depending ontemperature.

In order to suppress the occurrence of the reverse twist of liquidcrystal molecules to thereby retain the display quality of liquidcrystal display elements, it has been very usually carried out to add aslight quantity of an optically active substance having a clockwise orcounterclockwise helical twist sense. However, there is a problem thatsince the threshold voltage of liquid crystal compositions still yet hasa considerable temperature dependency, reduction in the display qualitydue to ambient temperature change is unavoidable.

With respect to improvements in the angle of view and contrast,considerable improvements have been made by employing supertwistedbirefringence effect mode (abbreviated to SBE). SBE mode is different incertain points from TN mode. Firstly, according to TN mode, addition ofa slight quantity of an optically active substance helps the glasssubstrates subjected to aligning treatment to twist liquid crystalmolecules by 90° between substrate due to their anchoring action. Here,the ratio (P/d) of the intrinsic helical pitch (P) of liquid crystalcomposition to the cell thickness (d) of display element is usuallyabout 10˜20. According to SBE mode, however, by increasing the quantityof an optically active substance added to a large extent to thereby makethe value of P/d 2 or less, the liquid crystal molecule is twisted by270°. Further, according to TN mode, liquid crystal molecules arealigned in a state where no voltage is impressed, inside the displayelement, so as to give an angle of liquid crystal molecule made againstglass substrates (pre-tilt angle) within several degrees, whereasaccording to SBE mode, alignment is made so as to give a pre-tilt angleof about 20°. An example of having improved the angle of view andcontrast according to such SBE mode has been reported (T.J. Scheffer, J.Nerhring, M. Kaufmann, H. Amstutz, D. Heimgartner and P. Eglin, Societyfor Information Display, 1985, International Symposium).

However, this SBE mode, too, raises a problem. Namely, since theintrinsic helical pitch P varies depending on the temperature change, itoccurs that when the value of P/d exceeds 2, 270° twist is changed to90° twist; hence it is necessary to keep the intrinsic helical pitch Pat a constant value irrespective of temperature.

Further, with respect to improvement in the display capacity, it isnecessary to improve the steepness of change in the transmittance in thecase where a voltage is impressed to the display element. G. Bauer andW. Fehrenbach reported a calculation result that the steepness isimproved to a large extent at 270° twist (15. Freiburger ArbeitstagungFlussigkristalle (1985)). In this case, too, it is necessary to be freefrom change in the intrinsic helical pitch depending on the temperaturechange.

With respect to the improvement in the response speed, Nakagawa andMasuda reported that response speed has been improved by employing adouble-layered guest-host mode (abbreviated to DGH mode) wherein twopieces of a liquid crystal display element of guest-host type are placedon one another and liquid crystal compositions having P/d=1.0 (i.e. 360°twist) are employed. (Nakagawa and Masuda, Society for InformationDisplay, 1985 International Symposium). In this case, too, it isimportant to be free from change in the intrinsic helical pitchdepending on the temperature change.

Further, in the display elements of the phase transition mode (PC mode),too, it is better to be free from change in the intrinsic helical pitchdepending on the temperature change. Further, with respect to overcomingreduction in the display quality depending on ambient temperaturechange, this may be effected by reducing the temperature dependency ofthe threshold voltage V_(th).

As to the cause of change in the threshold voltage V_(th) depending onthe temperature range, changes in the elastic constant of nematic liquidcrystals, the dielectric anisotropy, etc. depending on the temperaturechange, change in the intrinsic helical pitch depending on thetemperature change, etc. are enumerated. In order to improve thetemperature dependency of the threshold voltage, certain attempts havebeen made, and among these, a process of improving the temperaturedependency of the threshold voltage by controlling change in theintrinsic helical pitch depending on the temperature change has oftenbeen carried out.

When an optically active substance is added to a nematic liquid crystal,there is the following equation (1) between the concentration C of theoptically active substance and the intrinsic helical pitch P of theresulting liquid crystal composition, and in addition, the reciprocal ofthe intrinsic helical pitch P⁻¹ is also referred to as "twistability"and exhibits the strength of twist:

    P.sup.-1 =h·C                                     (1)

wherein h is referred to as helical twisting power and a constantintrinsic of the optically active substance and varies depending ontemperature. The change of h depending on the temperature change isexpressed by the following equation (2):

    h=α+βT+γT.sup.2 +- - -                    (2)

wherein α, β, γ, - - - each represent a proportionality factor.

An example of the twistability (P⁻¹) dependency of the threshold voltageV_(th) in the case where the temperature is constant and also the cellthickness of the TN type liquid crystal element is constant, is shown inFIG. 1. FIG. 1 shows the relationship between the twistability (P⁻¹) andthe threshold voltage V_(th) in the case where an optically activesubstance C-1 expressed by ##STR3## is added to a nematic liquidcomposition A shown below: ##STR4##

As seen from FIG. 1, the threshold voltage V_(th) rises with an increaseof the twistability (P⁻¹). Namely, the longer the intrinsic helicalpitch P of the liquid crystal composition becomes, the more thethreshold voltage V_(th) is reduced.

Further, the temperature dependency of the twistability (P⁻¹) in thecase where the optically active substance C-1 is added in 0.4% by weightto the above nematic liquid crystal composition A is shown in FIG. 2. Asseen from FIG. 2, the twistability (P⁻¹) decrease with temperature rise,and the intrinsic helical pitch P of the liquid crystal compositionincrease with temperature rise.

On the other hand, the temperature dependency of the threshold voltageV_(th) is shown in FIG. 3. The threshold voltage V_(th) lowers withtemperature rise. This indicates that as seen from FIG. 1 and FIG. 2,the intrinsic helical pitch P of the liquid crystal compositionincreases with temperature increase to thereby lower the thresholdvoltage V_(th). Further, it has been known that the threshold voltageV_(th) lowers depending on decrease in the elastic constant of thecomposition due to the temperature rise.

Thus, in order to reduce the temperature dependency of the thresholdvoltage V_(th), the intrinsic helical pitch P of the liquid crystalcomposition is preferred to be shorter with the temperature rise.

As apparent from the foregoing, control of the temperature dependency ofthe intrinsic helical pitch is very important for overcoming variousproblems raised on the liquid crystal display elements of variousdisplay modes. Namely, as to SBE mode, DGH mode and PC mode, theintrinsic helical pitch has been required to be constant irrespective oftemperature. Further, in order to improve the temperature dependency ofthe threshold voltage in the case of TN mode, the intrinsic helicalpitch has been required to be shorter with the temperature rise.However, too steep reduction in the cholesteric pitch with temperaturerise is not always satisfactory; thus it is also necessary to adjust theextent of the change of the intrinsic helical pitch depending on thetemperature change. However, if a generally known optically activesubstance is added, the intrinsic helical pitch of the resulting nematicliquid crystal composition increases with the temperature rise. Inshort, the twistability (P⁻¹) decreases with temperature rise; hence,even if the substance alone is added, it is impossible to control thetemperature dependency of the intrinsic helical pitch. That is, it isimpossible to be free from the temperature dependency of the intrinsichelical pitch or to obtain a temperature dependency which is contrary tothe conventional one.

When a plurality of optically active substances are added to nematicliquid crystals, the intrinsic helical pitch P_(Mix) of the resultingliquid crystal composition is expressed by the following equation (3):##EQU1##

This equation (3) indicates that the P_(Mix) ⁻¹ of the final liquidcrystal composition is the sum of the respective P_(i) ⁻¹ s obtainedwhen the respective optically active substances are singly added to theoriginal nematic liquid crystals in a concentration of C_(i).

In addition, when the symbol h of helical twisting power is madepositive relative to right-twisted optically active substance and madenegative relative to left-twisted optically active substance, theintrinsic helical pitch P_(Mix) of the liquid crystal compositionobtained by adding the mixture of right-twisted and left-twistedoptically active substances to nematic liquid crystals is also expressedby the equation (3).

In the case of conventional optically active substances, even if twooptically active substances each having a twist in the same sense aremixed and the mixture is added to nematic liquid crystals, the resultingtemperature dependency of the intrinsic helical pitch is nothing but anintermediate one between the two dependencies derived from therespective optically active substances; thus it is impossible to be freefrom the temperature dependency or to obtain a temperature dependencywhich is contrary to the conventional one. Now, it has been reportedthat when an optically active substance having a helical twist rightsense is mixed with that having a helical twist left sense in a definiteproportion and the mixture is added to nematic liquid crystals, then itis possible to be free from the temperature dependency of the intrinsichelical pitch or to obtain a temperature dependency which is contrary tothe conventional one (e.g. see U.S. Pat. No. 4,264,148, issued Apr. 28,1981). In this case, however, an optically active substance having righttwist and that having left twist are mixed so as to compensate thesetwists relative to one another to thereby obtain a definite intrinsichelical pitch; hence there is a case where the twistability (P⁻¹)becomes zero even in the vicinity of room temperature, depending on themixing proportions, and above and below this temperature, the twistingsenses are reverse to one another to thereby notably reduce the displayquality of liquid crystal display elements using a liquid crystalcomposition of this type. Thus, only a considerably limited range of themixing proportion will be employed. Further since the change of theintrinsic helical pitch is notable due to a slight difference of themixing proportion, the temperature control of the intrinsic helicalpitch is considerably difficult.

Further, since both a right twist optically active substance and a lefttwist one are added, one cannot help increasing the quantities thereofadded, in order to obtain a desired helical pitch. Thus, thecharacteristics of the resulting nematic liquid crystal composition suchas transition point, viscosity, threshold voltage V_(th), etc. changeconsiderably from the characteristics of the original nematic liquidcrystals. Further, since optically active substances are expensive, thefinal liquid crystal composition is also expensive. On account of thesedrawbacks, the practical use of a liquid crystal composition havingadded such two kinds of right twist and left twist optically activesubstances has been notably restricted.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a liquid crystalcomposition having a temperature dependency of the twistability (P⁻¹)which is contrary to the conventional one and increases with temperaturerise.

A second object of the present invention is to provide a nematic liquidcrystal composition almost free from the temperature dependency of thetwistability (P⁻¹).

A third object of the present invention is to provide a liquid crystaldisplay element having inhibited reduction in display quality due toambient temperature change and also having an improved angle of view,contrast, response speed, etc. in TN mode, SBE mode or DGH mode.

The present inventors have made extensive research on the temperaturedependency of the intrinsic helical pitch of liquid crystal compositionsinduced by singly adding various optically active substances to nematicliquid crystals. As a result, the present inventors have found that sofar known optically active substances reduce the reciprocal of helicalpitch P⁻¹ of the resulting liquid crystal composition with temperaturerise (that is, the temperature dependency is negative), whereas utterlycontrarily to the above, there exist optically active substances which,when singly added to nematic liquid crystals, increase twistability P⁻¹of the induced cholesteric phase in the resulting liquid crystalcomposition with temperature rise (that is, the temperature dependencyis positive). Further the present inventors have also found that when anoptically active substance which makes the temperature dependency of P⁻¹positive is mixed with an optically active substance which has the samehelical twist sense as that of the former substance and makes thetemperature dependency negative, and the mixture is added to nematicliquid crystals, it is possible to optionally control the temperaturedependency of the intrinsic helical pitch of the resulting liquidcrystal composition.

The present invention in a first aspect resides in

(1) a nematic liquid crystal composition which comprises (i) anoptically active substance or a mixture of such substances which are thesame in the helical twist sense to one another and which substance ormixture of substances make positive the temperature dependency of thetwistability in terms of the reciprocal of the intrinsic helical pitchthereof, of the cholesteric phase induced when singly added to at leastone member of nematic liquid crystals, and (ii) at least one member ofnematic liquid crystals, to thereby notably increase the twistabilitywith temperature rise.

The embodiments of the item (1) are shown in the following items (2) to(6):

(2) a nematic liquid crystal composition as an optically activesubstance according to the item (1), wherein said optically activesubstances are selected from the group consisting of compounds expressedby the formula (Ia), those expressed by the formula (Ib) and thoseexpressed by the formula (Ic), and have the optically active ##STR5##group or the ##STR6## group: ##STR7##

In the above formula (Ia), ##STR8## each independently represent benzenering, cyclohexane ring, dioxane ring, pyrimidine ring or pyridine ring;

l and m each represent an integer of 0, 1 or 2;

n represents an integer of 1 or 2;

the total value of (l+m+n) is 1 to 4;

X represents a single bond, ##STR9## --CH₂ -- or --CH₂ CH₂ --; Yrepresents a single bond when l=0;

Z represents a single bond when m=0;

Y and Z each independently represent ##STR10## --CH₂ O--, --OCH₂ --,--CH₂ CH₂ --, --CH═N-- or --N═CH-- when l·m≠0;

R represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms or cyano group;

R¹ represents a linear chain alkyl group of 2 to 10 carbon atoms; and

X represents a single bond when R is cyano group.

In the above formula (Ib), ##STR11## each independently representbenzene ring, cyclohexane ring, dioxane ring, pyrimidine ring orpyridine ring;

x and z each represent an integer of 0 or 1;

y represents an integer of 0, 1 or 2;

the total value of (x+y+z) is 0˜2;

X₁ represents a single bond, ##STR12## --CH₂ O-- or --CH₂ CH₂ --; Y₁represents a single bond when y=0, and Y₁ represents ##STR13## --CH₂O--, --OCH₂ --, --CH₂ CH₂ --, --CH═N-- or --N═CH-- when y is 1 or 2;

Z₁ represents a single bond, ##STR14## --CH₂ O--, --OCH₂ --, --CH₂ CH₂--, --CH═N-- or --N═CH--;

T¹, T², T³ and T⁴ each independently represent hydrogen atom, a halogenatom or cyano group;

R² represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms or cyano group or a halogen atom, and X₁ represents a single bondwhen R² is cyano group or a halogen atom; and

R¹ represents a linear chain alkyl group of 2 to 10 carbon atoms.

In the above formula (Ic), ##STR15## each independently representbenzene ring, cyclohexane ring, dioxane ring, pyrimidine ring orpyridine ring;

x and z each represent an integer of 0 or 1;

y represents an integer of 0, 1 or 2;

the total value of (x+y+z) is 0 to 2;

Y₁ represents a single bond when y=0, and

Y₁ represents ##STR16## --CH₂ O--, --OCH₂ --, --CH₂ CH₂ --, --CH═N-- or--N═CH-- when y is 1 or 2;

Z₁ represents a single bond, ##STR17## --CH₂ O--, --OCH₂ --, --CH₂ CH₂--, --CH═N-- or --N═CH--;

T¹, T², T³ and T⁴ each independently represent hydrogen atom, a halogenatom or cyano group; and

R¹ and R³ each independently represent a linear chain alkyl group of 2to 10 carbon atoms.

(3) A nematic liquid crystal composition according to the item (2)wherein said optically active substances selected from compoundsexpressed by the formula (Ia), compounds expressed by the formula (Ib)and compounds expressed by the formula (Ic) are contained in a quantityin the range of 0.05 to 10% by weight in said composition.

(4) A nematic liquid crystal composition according to the item (2)wherein said optically active substances are expressed by the formula(III) ##STR18## wherein a represents an integer of 0, 1 or 2; brepresents an integer of 1 or 2; the value of (a+b) is 2 or 3; V₁represents a single bond when a=0, and represents --COO--, --OCO--,--CH₂ O--, --OCH₂ -- or --CH₂ CH₂ -- when a is 1 or 2; and R and R¹ areas defined above.

(5) A nematic liquid crystal composition according to the item (2)wherein said optically active substances are expressed by the formula(IV) ##STR19## wherein c and d each represent an integer of 0 or 1; V₂represents a single bond, --COO-- or --OCO--; T¹, T², T³ and T⁴, eachrepresent hydrogen atom, halogen atom or cyano group; and R¹ and R² eachare as defined above.

(6) A nematic liquid crystal composition according to the item (2)wherein said optically active substances are expressed by the formula(V) ##STR20## wherein e represents an integer of 0, 1 or 2; f representsan integer of 1 or 2; the value of (e+f) is 1 to 3; V₃ represents asingle bond when e=0, and represents --COO-- or --CH₂ O-- when e is 1 or2; and R¹ and R³ are as defined above.

The present invention in a second aspect resides in (7) a nematic liquidcrystal composition which comprises (i) at least one member of opticallyactive substances which make positive the temperature dependency of thetwistability in terms of the reciprocal of the intrinsic helical pitchthereof, of the cholesteric phase induced when singly added to at leastone member of nematic liquid crystals, (ii) at least one member ofoptically active substances which have the same helical twist sense asthat of the former optically active substances and make negative thetemperature dependency of the twistability of the induced cholestericphase when singly added to at least one member of nematic liquidcrystals, and (iii) at least one member of nematic liquid crystals.

The embodiments of the item (7) are shown in the following items (8) to(17):

(8) A nematic liquid crystal composition according to the item (7),wherein said optically active substances (i) which make positive thetemperature dependency of the twistability of the cholesteric phaseinduced when singly added to at least one member of nematic liquidcrystals are compounds selected from the group consisting of compoundsexpressed by the following formula (Ia), compounds expressed by thefollowing formula (Ib) and compounds expressed by the following formula(Ic), and said optically active substances (ii) which have the samehelical sense as that of the former optically active substances andwhich make negative the temperature dependency of the twistability ofthe cholesteric phase induced when singly added to at least one memberof nematic liquid crystals are compounds expressed by the followingformula (II): ##STR21##

In the above formula (Ia), ##STR22## each independently representbenzene ring, cyclohexane ring, dioxane ring, pyrimidine ring orpyridine ring;

l and m each represent an integer of 0, 1 or 2;

n represents an integer of 1 or 2;

the value of (l+m+n) is 1 to 4;

X represents a single bond, ##STR23## --CH₂ -- or --CH₂ CH₂ --; Yrepresents a single bond when l=0, and Z represents a single bond whenm=0, and Y and Z each independently represent ##STR24## --CH₂ O--,--OCH₂ --, --CH₂ CH₂ --, --CH═N-- or --N═CH-- when l·m≠0;

R represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms or cyano group;

R¹ represents a linear chain alkyl group of 2 to 10 carbon atoms; and

X represents a single bond when R is cyano group.

In the above formula (Ib), ##STR25## each independently representbenzene ring, cyclohexane ring, dioxane ring, pyrimidine ring orpyridine ring;

x and z each represent an integer of 0 or 1;

y represents an integer of 0, 1 or 2;

the value of (x+y+z) is 0 to 2;

X₁ represents a single bond, ##STR26## --CH₂ O or --CH₂ CH₂ --; Y₁represents a single bond when y=0 and represents ##STR27## --CH₂ O--,--OCH₂ --, --CH₂ CH₂ --, --CH═N-- or --N═CH-- when y is 1 or 2;

Z₁ represents a single bond, ##STR28## --CH₂ O--, --OCH₂ --, --CH₂ CH₂--, --CH═N-- or --N═CH--;

T¹, T², T³ and T⁴ each independently represent hydrogen atom, a halogenatom or cyano group;

R² represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms or cyano group or a halogen atom, and when R² is cyano group or ahalogen atom, X₁ represents a single bond; and

R¹ represents a linear chain alkyl group of 2 to 10 carbon atoms.

In the above formula (Ic), ##STR29## each independently representbenzene ring, cyclohexane ring, dioxane ring, pyrimidine ring orpyridine ring;

x and z each represent an integer of 0 or 1;

y represents an integer of 0, 1 or 2;

the value of (x+y+z) is 0 to 2;

Y₁ represents a single bond when y=0, and represents ##STR30## --CH₂O--, --OCH₂ --, --CH₂ CH₂ --, --CH═N-- or --N═CH-- when y is 1 or 2;

Z₁ represents a single bond, ##STR31## --CH₂ O--, --OCH₂ --, --CH₂ CH₂--, --CH═N-- or --N═CH--;

T¹, T², T³ and T⁴ each independently represent hydrogen atom, a halogenatom or cyano group; and

R¹ and R³ each independently represent a linear chain alkyl group of 2to 10 carbon atoms. ##STR32##

In the formula (II), ##STR33## each independently represent benzenering, cyclohexane ring, dioxane ring, pyrimidine ring or pyridine ring;

p and q each represent 0, 1 or 2;

r represents 1 or 2;

the value of (p+q+r) is 1 to 4;

s represents 0, 1, 2, 3 or 4;

X₂ represents --O--, --CO--, --COO--, --OCO--, --OCOO--, --OCH₂ -- or--OCH₂ CH₂ --;

Y₂ represents a single bond when p=0;

Z₂ represents a single bond when q=0;

Y₂ and Z₂ each independently represent --COO--, --OCO--, --CH₂ O--,--OCH₂ --, --CH₂ CH₂ --, --CH═N-- or --N═CH-- when p·q≠0;

W represents a single bond --COO-- or --OCO-- when s=0, and represents--O--, --COO-- or --OCO-- when s represents 1, 2, 3 or 4;

R⁴ represents an alkyl group of 1 to 15 carbon atoms or cyano group;

R⁵ represents a linear chain alkyl group of 2 to 10 carbon atoms; and

X₂ represents a single bond when R⁴ is cyano group.

(9) A nematic liquid crystal composition according to the item (8)wherein optically active substances selected from the group consistingof compounds expressed by the formula (Ia), compounds expressed by theformula (Ib) and compound expressed by the formula (Ic), together withoptically active substances expressed by the formula (II), are containedin a quantity of 0.05 to 10% by weight in the composition.

(10) A nematic liquid crystal composition according to the item (8)wherein said optically active substances which make positive thetemperature dependency of the twistability are compounds expressed bythe following formula (III) and said optically active substances whichhave the same helical twist sense as that of the former optically activesubstances and which make negative the temperature dependency of thetwistability are compounds expressed by the following formula (VI):##STR34## wherein a represents an integer of 0, 1 or 2;

b represents an integer of 1 or 2;

the value of (a+b) is 2 or 3;

V₁ represents a single bond when a=0 and represents --COO--, --OCO--,--CH₂ O, --OCH₂ -- or --CH₂ CH₂ -- when a is 1 or 2;

R represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms or cyano group; and

R¹ represents a linear chain alkyl group of 2 to 10 carbon atoms.##STR35## wherein g represents 0, 1 or 2;

h represents 1 or 2;

the value of (g+h) is 1 to 3;

V₄ represents a single bond when g=0 and represents --COO--, --OCO--,--CH₂ O-- or --OCH₂ -- when g is 1 or 2;

W₁ represents a single bond, --O-- or --COO--; and

R⁶ represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms or cyano group.

(11) A nematic liquid crystal composition according to the item (8)wherein said optically active substances which make positive thetemperature dependency of the twistability are compounds expressed bythe following formula (III) and said optically active substances whichhave the same helical twist sense as that of the former optically activesubstances and which make negative the temperature dependency of thetwistability are compounds expressed by the following formula (VII):##STR36## wherein b represents an integer of 1 or 2;

the value of (a+b) is 2 or 3;

V₁ represents a single bond when a=0 and represents --COO--, --OCO--,--CH₂ O--, --OCH₂ -- or --CH₂ CH₂ -- when a is 1 or 2;

R represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms or cyano group; and

R¹ represents a linear chain alkyl group of 2 to 10 carbon atoms.##STR37## wherein i represents 0, 1 or 2;

j represents 1 or 2;

the value of (i+j) is 1 to 3;

V₅ represents a single bond when i=0 and represents --COO--, --OCO--,--CH₂ O-- or --OCH₂ -- when i is 1 or 2;

R⁷ represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms; and

R⁸ represents a linear chain alkyl group of 2 to 10 carbon atoms.

(12) A nematic liquid crystal composition according to the item (8)wherein said optically active substances which make positive thetemperature dependency of the twistability are compounds expressed bythe following formula (IV) and said optically active substances whichhave the same helical twist sense as that of the former optically activesubstances and which make negative the temperature dependency of thetwistability are compounds expressed by the following formula (VI):##STR38## wherein c and d each represent an integer of 0 or 1;

V₂ represents a single bond, --COO-- or --OCO--;

T¹, T², T³ and T⁴ each represent hydrogen atom, halogen atom or cyanogroup;

R¹ represents a linear chain alkyl group of 2 to 10 carbon atoms; and

R² represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms, cyano group or halogen atom. ##STR39## wherein g represents 0, 1or 2;

h represents 1 or 2;

the value of (g+h) is 1 to 3;

V₄ represents a single bond when g=0 and represents --COO--, --OCO--,--CH₂ O-- or --OCH₂ -- when g is 1 or 2;

W₁ represents a single bond, --O-- or --COO--; and

R⁶ represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms or cyano group.

(13) A nematic liquid crystal composition according to the item (8)wherein said optically active substances which make positive thetemperature dependency of the twistability are compounds expressed bythe following formula (IV) and said optically active substances whichhave the same helical twist sense as that of the former optically activesubstances and make negative the temperature dependency of thetwistability are compounds expressed by the following formula (VII):##STR40## wherein c and d each represent an integer of 0 or 1;

V₂ represents a single bond, --COO-- or --OCO--;

T¹, T², T³ and T⁴ each represent hydrogen atom, halogen atom or cyanogroup;

R¹ represents a linear chain alkyl group of 2 to 10 carbon atoms; and

R² represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms, cyano group or halogen atom. ##STR41## wherein i represents 0, 1or 2;

j represents 1 or 2;

the value of (i+j) is 1 to 3;

V₅ represents a single bond when i=0 and represents --COO--, --OCO--,--CH₂ O--, or --OCH₂ -- when i is 1 or 2;

R⁷ represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms; and

R⁸ represents a linear chain alkyl group of 2 to 10 carbon atoms.

(14) A nematic liquid crystal composition according to the item (8)wherein said optically active substances which make positive thetemperature dependency of the twistability are compounds expressed bythe following formula (V) and said optically active substances whichhave the same helical twist sense as that of the former optically activesubstances and which make negative the temperature dependency of thetwistability are compounds expressed by the following formula (VI):##STR42## wherein e represents an integer of 0, 1 or 2;

f represents an integer of 1 or 2;

the value of (e+f) is 1 to 3;

V₃ represents a single bond when e=0 and represents --COO-- or --CH₂ O--when e is 1 or 2; and

R¹ and R³ each independently represent a linear chain alkyl group of 2to 10 carbon atoms. ##STR43## wherein g represents 0, 1 or 2;

h represents 1 or 2;

the value of (g+h) is 1 to 3;

V₄ represents a single bond when g=0 and represents --COO--, --OCO--,--CH₂ O-- or --OCH₂ -- when g is 1 or 2;

W₁ represents a single bond, --O--, or --COO--; and

R⁶ represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms or cyano group.

(15) A nematic liquid crystal composition according to the item (8)wherein said optically active substances which make positive thetemperature dependency of the twistability are compounds expressed bythe following formula (V) and said optically active substances whichhave the same helical twist sense as that of the former optically activesubstances and which make negative the temperature dependency of thetwistability are compounds expressed by the following formula (VII):##STR44## wherein e represents an integer of 0, 1 or 2;

f represents an integer of 1 or 2;

the value of (e+f) is 1 to 3;

V₃ represents a single bond when e=0 and represents --COO-- or --CH₂ Owhen e is 1 or 2; and

R¹ and R³ each independently represents a linear chain alkyl group of 2to 10 carbon atoms. ##STR45## wherein i represents 0, 1 or 2;

j represents 1 or 2;

the value of (i+j) is 1 to 3;

V₅ represents a single bond when i=0 and represents --COO--, --OCO--,--CH₂ O or --OCH₂ -- when j is 1 or 2;

R⁷ represents an alkyl group or an alkoxy group each of 1 to 15 carbonatoms; and

R⁸ represents a linear chain alkyl group of 2 to 18 carbon atoms.

(16) A nematic liquid crystal composition according to the item (8)wherein said optically active substances are selected so that thetwistability of the induced cholesteric phase can be constantirrespective of temperature change in a definite temperature range.

(17) A nematic liquid crystal composition according to claim 8 whereinsaid optically active substances are selected so that the temperaturedependency of the twistability of the induced cholesteric phase can havea desired value in a definite temperature range.

The present invention in a third aspect resides in the following items(18) and (19):

(18) A liquid crystal display element characterized by using a nematicliquid crystal composition which comprises (i) at least one member ofoptically active substances which are the same in the helical twistsense to one another and make positive the temperature dependency of thetwistability in terms of the reciprocal of the intrinsic helical pitchthereof, of the cholesteric phase induced when singly added to at leastone member of nematic liquid crystals, and (ii) at least one member ofnematic liquid crystals, to thereby notably increase the twistabilitywith temperature rise.

(19) A liquid crystal display element characterized by using a nematicliquid crystal composition which comprises (i) at least one member ofoptically active substances which make positive the temperaturedependency of the twistability in terms of the reciprocal of theintrinsic helical pitch thereof, of the cholesteric phase induced whensingly added to at least one member of nematic liquid crystals, (ii) atleast one member of optically active substances which have the samehelical twist sense as that of the former optically active substancesand make negative the temperature dependency of the twistability of thecholesteric phase induced when singly added to at least one member ofnematic liquid crystals, and (iii) at least one member of nematic liquidcrystals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a chart illustrating the twistability dependency of thethreshold voltage.

FIGS. 2, 4, 5, 6 and 12 respectively show a chart illustrating thetemperature dependency of the twistability.

FIGS. 3, 7, 8, 9, 10 and 17 respectively show a chart illustrating thetemperature dependency of the threshold voltage.

FIGS. 11, 13, 14, 15, 16, 18, 19, 20 and 21 respectively show a chartillustrating the normalized value of temperature dependency of thetwistability between 20° C. and 70° C.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be described referring to examples.

In FIG. 4 is shown the temperature dependency of twistability (P⁻¹) inthe case where an optically active substance B-1 of the formula##STR46## disclosed in Japanese patent application laid-open No. Sho61/43 (1986), which substance is obtained using R(-)-2-octanol as astarting raw material and has a right helical twist sense is added tothe aforementioned nematic liquid crystal composition A. For reference,the effect of the optically active substance C-1 shown in FIG. 2 is alsoshown therein. As seen from FIG. 4, the value of P⁻¹ in the case wherethe optically active substance B-1 is added increases with temperaturerise, and exhibits a contrary tendency to that of a monotonic reductionin the case of addition of the compound C-1.

In general, as a parameter for comparing the temperature dependency ofthe twistability of a liquid crystal material in the temperature rangebetween t₁ and t₂ will be employed the normalized value of ΔP⁻¹ (t₁ -t₂)expressed by the following equation specifying the above temperaturedependency: ##EQU2## wherein P⁻¹ (t) refers to the value of atwistability at a temperature of t° C. These two nematic liquid crystalcompositions were compared referring to the normalized values of thetemperature dependency ΔP⁻¹ (20˜40) of the twistablility specified bythe equation (4). As a result, the value of ΔP⁻¹ (20˜40) of thecomposition having the compound B-1 added as an optically activesubstance was 1.93, whereas that of the composition having the compoundC-1 was -0.48.

Further, a liquid crystal composition N and a liquid crystal compositionC were respectively prepared by adding the compound B-1 (one part byweight) to the above liquid crystal composition (A) (100 parts byweight) and adding the compound C-1 (0.1 part by weight) to thecomposition (A) (100 parts by weight), and the resulting compositionswere respectively sealed in a TN cell having a definite distance betweenthe electrodes to compare the temperature dependencies of the thresholdvoltages (V_(th)). The results are shown in FIG. 3. As seen from theabove example, the liquid crystal composition of the present inventioncontaining the optically active substance B-1 has an improvedtemperature dependency of V_(th), and in particular, the reduction ofV_(th) is small in the high temperature region of nematic phasetemperature range.

As an example wherein two optically active substances each having anopposite helical twist sense to one another were added, a nematic liquidcrystal composition M was prepared by adding the optically activesubstance C-1 having a right helical twist (2.0 parts by weight) and anoptically active compound C-2 expressed by the formula ##STR47## andhaving a left helical twist (1.5 part by weight) together to the aboveliquid crystal composition (A) (100 parts by weight), and thecharacteristics of this composition M were compared with those of theabove liquid crystal composition N. The results are shown in Table 1.For reference, the characteristics of the nematic liquid compositions Aand C are also shown therein.

                  TABLE 1                                                         ______________________________________                                                       Viscosity                                                              NI point                                                                             at 20° C.                                                                        ΔP.sup.-1                                                                        ΔV.sub.th /                                   (°C.)                                                                         (cp)      (20˜40)                                                                          Δt (0˜40)*                      ______________________________________                                        Composition M                                                                           70.2     27.8      1.49   -7.3                                      Composition N                                                                           71.5     25.6      1.93   -6.0                                      Composition C                                                                           71.8     25.8      -0.48  -9.3                                      Composition A                                                                           72.4     25.2                                                       ______________________________________                                         *ΔV.sub.th /Δt refers to a parameter expressing the               temperature dependency of threshold voltage defined in the equation (5) o     Example 1 described later.                                               

As seen from Table 1, since the composition N has a small quantity ofthe optically active substance added, its upper limit temperature (N-Ipoint) of the nematic phase is not reduced so much from that of theoriginal composition A, and also the viscosity rise is restricted to asmall value. On the other hand, since the composition M has a largequantity of the optically active substance added, its N-I point lowersby 2° C. or more, and also its viscosity rise is much higher than thatof the composition N. Further, in comparison of the temperaturedependency of the threshold voltage, it is seen that the compositions Nand M each having a positive ΔP⁻¹ are smaller in |ΔV_(th) /Δt| ascompared with the composition C having a negative ΔP⁻¹. Further, incomparison of the composition N with the composition M, declination inthe ΔV_(th) /Δt of the composition N is smaller than that of thecomposition M.

As described above, the composition of the present invention iscollectively superior as a nematic liquid crystal composition having apositive temperature dependency of the twistability.

Examples of the optically active substances which are preferred as thecomponent of the liquid crystal composition of the present invention arecompounds expressed by the above-mentioned formulas (III)-(V). These arecharacterized by having an optically active group of ##STR48## As to thestructure of optically active groups, there are some kinds, and as towell-known ##STR49## among raw material alcohols for introducingoptically active groups, only the one having an absolute configurationof sinister type (S type) is existent in the natural world, and therehas still been no example of its optical resolution; hence in the caseof derivatives of such an alcohol, there is little room to choose thoseof right twist and left twist. Whereas, in the case of ##STR50## as theraw material for the optically active substances constituting acomponent of the composition of the present invention, the opticalresolution thereof is easy and two kinds of optical isomers having anabsolute configuration of S type and that of rectus type (R type),respectively are obtained; hence the freeness of choice in the form ofoptically active substances is large. In other words, two kinds ofcompounds having all the same structural formula but being differentonly in the steric configuration of the optically active group areproduced to thereby obtain two kinds of optically active substanceshaving all the same characteristics but having right and left twistsenses, respectively. Further, adequate, separate use of such two kindsof optically active substances brings about an advantage that two kindsof compositions having all the same characteristics but having right andleft twist senses, respectively are obtained.

Next, the above-mentioned optically active substance B-1 (0.43 wt. %)and an optically active substance expressed by the following formula C-3(0.57 wt. %): ##STR51## were mixed with the above-mentioned nematicliquid crystal composition (A). The temperature dependency of thetwistability (P⁻¹) of the resulting liquid crystal composition is shownin FIG. 5. In FIG. 5, (Mix 1) shows the temperature dependency of thetwistability (P⁻¹) in the case where optically active substances B-1 andC-3 were mixed and added to the composition (A), (B-1) shows that in thecase where B-1 alone (0.43 wt. %) was added to the composition, and(C-3) shows that in the case where the compound C-3 alone (0.57 wt. %)was added thereto. As seen from FIG. 5, there is almost no change in P⁻¹depending on temperature in the case of (Mix 1).

Further, the optically active substance B-1 (0.8 wt. %) and theoptically active substance C-3 (0.2 wt. %) were added together to thenematic liquid crystal composition (A). The temperature dependency ofthe twistability (P⁻¹) of the resulting composition is shown as (Mix 2)of FIG. 5. Further, the case where the optically active substance B-1alone (1.0 wt. %) was added is shown as (Mix 3) of FIG. 5.

In the cases of (Mix 2) and (Mix 3), it is seen that the twistability(P⁻¹) steeply increases with temperature rise. Further, the larger theproportion of B-1, the steeper the change in the twistability (P⁻¹).This indicates that by varying the mixing proportions of B-1 and C-3, itis possible to continuously and optionally control the temperaturedependency of the twistability (P⁻¹) from its nearly constant state upto its steeply ascending state with temperature rise.

Further, the following optically active substance B-2 having a lefthelical twist sense, obtained by using as its starting raw material,S(+)-2-octanol disclosed in Japanese patent application laid-open No.Sho 61-43 (1986) (0.48 wt. %): ##STR52## and the following opticallyactive substance C-4 (0.52 wt. %): ##STR53## were mixed and added to theabove nematic liquid crystal composition (A). The temperature dependencyof the twistability (P⁻¹) of the resulting composition is shown in FIG.6.

In FIG. 6, (Mix 4) shows the case where the optically active substancesB-2 (0.48 wt. %) and C-4 (0.52 wt. %) were mixed and added; (B-2) showsthe case where the compound B-2 alone (0.48 wt. %) was added; and (C-4)shows the case where the compound C-4 alone (0.52 wt. %) was added; thus(Mix 4) has still almost no change in P⁻¹ depending on temperature.

Further, the optically active substances B-2 (0.8 wt. %) and C-4 (0.2wt. %) were mixed and added to the nematic liquid crystal composition(A). The temperature dependency of the twistability (P⁻¹) of theresulting composition is shown as (Mix 5) of FIG. 6. Further the casewhere the optically active substance B-2 alone (1.0 wt. %) was added isshown as (Mix 6) of FIG. 6.

In comparison of (Mix 5) with (Mix 6), the twistability (P⁻¹) steeplyincreases with temperature rise, and as the proportion of B-2 increases,change in the twistability (P⁻¹) becomes steep. This still indicatesthat by varying the mixing proportions of B-2 and C-4, it is possible tocontinuously and optionally control the temperature dependency of thetwistability (P⁻¹) from its almost constant state up to a steeplyascending state with temperature rise.

As seen from the foregoing, if an optically active substance having aterminal group, ##STR54## as shown in the general formula (Ia), thegeneral formula (Ib) or the general formula (Ic) is used, either even inthe case where the substance is added together with another opticallyactive substance having a larger twistability (P⁻¹) at room temperatureand the same helical twist sense (see FIG. 5), or contrarily even in thecase where the substance is added together with another optically activesubstance having a smaller twistability (P⁻¹) at room temperature andthe same helical twist sense (see FIG. 6), it is possible to optionallycontrol the change in the intrinsic helical pitch depending ontemperature change, of the resulting compositions. More details will bedescribed later in Examples).

As the compounds expressed by the above general formulas (Ia), (Ib) and(Ic) among the optically active substances used as a componentconstituting the liquid crystal composition of the present invention,compounds expressed by the above formulas (III), (IV) and (V) and havinga 1-methyl-alkyloxy group as an optically active group are preferred.Further, as the compounds expressed by the above formula (II) as theother optically active substances, compounds expressed by the aboveformulas (VI) and (VII) are suitable.

These compounds can be prepared for example as follows:

(1) Compounds of the formula (III) wherein V₁ represents --OCO-- may beprepared according to the following scheme 1 (see Japanese patentapplication laid-open No. Sho 61-43): ##STR55##

(2) Compounds of the formula (III) wherein V₁ represents --COO-- may--COO-- may be prepared according to the following scheme 2: ##STR56##

(3) Compounds of the formula (III) wherein V₁ represents --CH₂ O-- or--OCH₂ -- may be prepared according to the process disclosed in Japanesepatent application laid-open No. Sho 61-63633.

(4) Compounds of the formula (IV) wherein V₂ represents --COO--; drepresents zero; T¹, T² and T³ each represent hydrogen atom; and T⁴represents a halogen atom may be prepared according to the followingscheme 3.

Further, compounds of the formula (IV) wherein V₂ represents --COO--;either one of T¹ or T² represents a halogen atom and the other, T³ andT⁴ each represent hydrogen atom may be prepared according to thefollowing scheme 4 (Japanese patent application No. Sho 61-51512, notyet laid-open). ##STR57##

(5) Compounds of the formula (V) wherein V₃ represents --COO-- may beprepared according to the following scheme 5: ##STR58##

(6) Compounds of the formula (V) wherein e=0, f=2 and V₃ represents asingle bond may be prepared according to the following scheme 6:##STR59##

(7) Compounds of the formula (VII) wherein V₅ represents --OCO-- may beprepared according to the process disclosed in Japanese patentapplication laid-open No. Sho 60-149548/1985 (scheme 7): ##STR60##

Compounds of the formula (VI) wherein V₄ represents --OCO-- and W₁represents --COO-- may be prepared using an optically active alcohol##STR61## in the same manner as in the preparation of the scheme 7.

Certain compounds expressed by the formula (VI) or (VII) arecommercially available. Some of these are illustrated in the followingTable 2.

                                      TABLE 2                                     __________________________________________________________________________    Producer & Trade                                                              Designation  Structural formula                                               __________________________________________________________________________    BDH Chemicals                                                                          CB-15                                                                              ##STR62##                                                       BDH Chemicals                                                                          C-15                                                                               ##STR63##                                                       Merck & Co.                                                                            S 1082                                                                             ##STR64##                                                       Merck & Co.                                                                            S 811                                                                              ##STR65##                                                       Chisso Corp.                                                                           CM-19                                                                              ##STR66##                                                       Chisso Corp.                                                                           CM-20                                                                              ##STR67##                                                       Chisso Corp.                                                                           ER-M                                                                               ##STR68##                                                       __________________________________________________________________________

In addition, from ##STR69## as a raw material for optically activesubstances expressed by the formula (III) and being a componentconstituting the composition of the present invention are, also preparedcompounds expressed by the above formula (VII). Such compounds arecharacterized by having an optically active group of ##STR70## and thetemperature depend (ΔP⁻¹) of the twistability of the cholesteric phaseinduced when singly added to nematic liquid crystals is negative.

Among compounds expressed by the formula (III) or the formula (VII), twokinds of isomers having the same structure but being different only inthe steric configuration of the optically active group are existent.These isomers are opposite in the helical twist sense to one another,but other characteristics thereof are same. By combining opticallyactive substances expressed by the formula (III) with those expressed bythe formula (VII) and adjusting the temperature dependency of P⁻¹ of theliquid crystal composition, it is possible to easily obtain two kinds ofnematic compositions being different only in the helical twist sense butbeing the same in the other characteristics.

For example, it is also possible to combine an optically activesubstance having a group ##STR71## and having a left twist sense,obtained using S(+)-2-octanol as a starting raw material, with anoptically active substance having a group ##STR72## and having a lefttwist sense, obtained using S(+)-2-octanol as a starting raw material.Further, it is also possible to combine an optically active substancehaving a group ##STR73## and having a right twist sense, obtained usingR(-)-2-octanol as a starting raw material, with an optically activesubstance having a group ##STR74## and having a right twist sense,obtained using R(-)-2-octanol as a starting raw material.

What is important in the present invention is that when a compoundselected from the group consisting of compounds expressed by theformulas (Ia), (Ib) and (Ic) is combined with a compound expressed bythe formula (II) and the mixture is added to a nematic liquid crystalcomposition to thereby control the temperature dependency of thetwistability, the respective helical twist senses of these two componentcompounds should be the same. Further, even when two or more compoundschosen from the group consisting of compounds expressed by the formulas(Ia), (Ib) and (Ic) are mixed and the mixture is added to a nematicliquid crystal composition, the respective twist senses of thesecompounds should be the same.

The helical twist senses of these optically active compounds may beconfirmed according to a known method such as contact method (see G. W.Gray and D. G. McDonnell, Mol. Cryst. Liq. Cryst., Vol. 34 (Letters),(1977), pp211).

The content of the optically active substances used as a component forthe liquid crystal composition of the present invention, in theresulting liquid crystal composition, is in the range of 0.05 to 10 wt.%, preferably 0.05 to 5 wt. %, in terms of the content of a compoundalone, in the case where a compound selected from the group consistingof compounds expressed by the formulas (Ia), (Ib) and (Ic) is singlyadded, or in terms of the content of the total of a compound selectedfrom the group consisting of compounds expressed by the formulas (Ia),(Ib) and (Ic) and a compound expressed by the formula (II). If thequantity of optically active substances added is less than 0.05 wt. %,it is impossible to adjust the helical pitch of the resulting liquidcrystal composition to desired length, while if it exceeds 10 wt. %, thenematic phase temperature range of the resulting liquid crystalcomposition is notably narrow.

Next, advantages brought about by the present invention will bedescribed.

(i) Since the liquid crystal composition of the present invention has anotably increased twistability with temperature rise, it is possible toobtain a liquid crystal display element having a small temperaturedependency of the threshold voltage by the use of the composition.

(ii) A liquid crystal composition of which the twistability is constantin a certain temperature range is easily prepared; hence by employingthis composition for displays of SBE mode, DGH mode, phase change modeand other modes, it is easy to obtain a liquid crystal display elementhaving a broad angle of view, a high contrast and a high response speed.

(iii) A liquid crystal composition having optionally controlled thechange of the twistability (P⁻¹) depending on temperature is easilyobtained; hence when it is applied to TN mode, a good liquid crystaldisplay element having a slight reduction in the display qualitydepending on the ambient temperature change is obtained.

(iv) In the case of the liquid crystal composition of the presentinvention, optically active substances each having the same helicaltwist sense are used; hence there occurs no reversal of twist of theliquid crystal molecules which is inherent of the composition containingoptically active substances of opposite twist senses to one another.

(v) Since optically active substances each having the same helical twistsense are used, a small quantity thereof added affords a desired pitchas compared with the case where an optically active substance having aright twist and that having a left twist are mixed and used.

(vi) Since a small quantity of the optically active substance issufficient to afford a desired pitch, the properties of the originalliquid crystal composition are not influenced so much by the additionthereof.

(vii) It is possible to produce a nematic composition relatively cheaplyby adding a small quantity of the optically active substances which aregenerally expensive as compared with nematic compounds.

(viii) Since optically active substances each having the same twistsense are used, it is unnecessary to limit the mixing proportion as inthe case where optically active substances each having a right twist anda left twist are mixed and used; hence it is easy to control thetemperature dependency of the intrinsic helical pitch.

In addition to the above effectiveness (i)˜(viii), the effectiveness ofthe present invention will be further described in the followingExamples.

The present invention will be described in more detail by way ofExamples, but it should not be construed to be limited thereto.

The helical pitch P described in Example was measured according to Canowedge method.

EXAMPLE 1

To a nematic liquid crystal composition (D) (100 parts by weight)consisting of ##STR75## was added a compound B-3 (one part by weight)expressed by the formula ##STR76## as an optically active substance,which compound is disclosed in Japanese patent application laid-open No.Sho 61-43 (1986); is obtained using R(-)-2-octanol as a starting rawmaterial; and has a right helical twist sense, to prepare a nematicliquid crystal composition. This composition was sealed in a cellprovided with substrates having the surface coated with polyvinylalcohol and subjected to rubbing treatment and having a cell gap of 9 μmto prepare a TN liquid crystal cell. The threshold voltage of this TNliquid crystal cell was measured at various temperatures. The resultsare shown in FIG. 7.

When the temperature dependency of the threshold voltage is shown by thevalue of ΔV_(th) /Δt expressed by the following equation (5): ##EQU3##wherein V_(th) (t) represents a threshold voltage at a temperature of t°C., the temperature dependency of the TN liquid crystal cell is shown inTable 3. It is believed that the dependency is due to the fact that thetemperature dependency ΔP⁻¹ of the twistability of the nematic liquidcrystal composition obtained by the addition of the compound B-3 has apositive value, as described later.

                  TABLE 3                                                         ______________________________________                                                         Ex. 1                                                                              Comp. ex. 1                                             ______________________________________                                        ΔV.sub.th /Δt                                                              -30˜25° C.                                                                      0.0   -3.8                                                      25˜80° C.                                                                     -5.5   -8.7                                            ______________________________________                                    

COMPARATIVE EXAMPLE 1

To the nematic liquid crystal composition (D) (100 parts by weight)shown in Example 1 was added the above optically active substance C-1(0.1 part by weight) to prepare a nematic liquid crystal composition,which was then sealed in the same cell as in Example 1 to measure thethreshold voltage. The results are shown in FIG. 7 together with thoseof Example 1. Further the temperature dependency of the thresholdvoltage is shown in Table 3.

EXAMPLE 2 AND COMPARATIVE EXAMPLE 2

To the nematic liquid crystal composition (D) (100 parts by weight) wasadded a compound B-4 (1.6 part by weight) expressed by the formula##STR77## as an optically active substance, which compound is alsodisclosed in the above Japanese patent application laid-open No. Sho61-43; is obtained using S(+)-2-octanol as a starting raw material; andhas a left helical twist sense, to prepare a nematic liquid crystalcomposition, which was then sealed in the same TN cell as in Example 1to measure the threshold voltage. The results are shown in FIG. 8.Further, from the results of FIG. 8 was calculated the temperaturedependency of the threshold voltage ΔV_(th) /Δt. The results are shownin Table 4.

As a comparative example, to the liquid crystal composition (D) (100parts by weight) was added cholesteryl nonanoate (0.2 part by weight) toprepare a nematic liquid crystal composition, from which was preparedthe same TN liquid crystal cell as in Example 2, followed by measuringits temperature dependency of the threshold voltage. The results areshown in FIG. 8 and Table 4 together with those of Example 2.

                  TABLE 4                                                         ______________________________________                                                         Ex. 2                                                                              Comp. ex. 2                                             ______________________________________                                        ΔV.sub.th /Δt                                                              -30˜25° C.                                                                     +3.3   -4.7                                                      25˜80° C.                                                                     -3.3   -6.5                                            ______________________________________                                    

In comparison of Example 2 with Comparative example 2, the temperaturedependency of the threshold voltage ΔV_(th) /Δt between 25° and 80° C.of Example 2 is about half of that of Comparative example 2. Further, onthe lower temperature side, the value of ΔV_(th) /Δt of Example 2 is apositive value contrarily to conventional temperature dependency of thethreshold voltage.

As apparent from FIG. 8, there is the maximum value of the thresholdvoltage V_(th) in the vicinity of 30° C. in the case of Example 2. Thisis considered to be due to the fact that on the lower temperature side,increase in the intrinsic helical pitch has a greater influence upon thetemperature change of V_(th) than the effects brought about by thetemperature change in the elastic constant and other properties of thecomposition, whereas on the higher temperature side, the effect broughtabout by the temperature change in the elastic constant is notable. Thefact that a maximum value appears in the threshold voltage as describedabove is a phenomenon which has never been observed in the so far knownliquid crystal compositions.

In the case of the prior art, as the temperature lowers, the thresholdvoltage V_(th) of the liquid crystal display element increasesmonotonously, and approaches a definite driving voltage up to a certainvalue; thus

along with the influence of increase in the viscosity of the liquidcrystal, the response speed of the display lowers.

Whereas, by using the composition of the present invention, it ispossible to inhibit the V_(th) reduction on the higher temperature side,and also it is possible to inhibit the V_(th) rise on the lowertemperature side; hence the difference between the definite drivingvoltage and the threshold voltage may be kept to a certain value overthe practical temperature range so that it is possible to compensate thelowering of the response speed due to the viscosity rise in the lowertemperature region. Further, since it is possible to reduce thetemperature dependency of the threshold voltage, that is, it is possibleto reduce the absolute value of |ΔV_(th) /Δt|, a superior display in theaspect of the contrast is obtained over a broad temperature range.

EXAMPLE 3

To the nematic liquid crystal composition (D) (100 parts by weight)shown in Example 1 was added a compound B-5 (0.25 part by weight)expressed by the formula ##STR78## as an optically active substance,which compound is obtained using R(-)-2-octanol as a starting rawmaterial and has a right helical twist sense, to prepare a nematicliquid crystal composition, which was then sealed in the same TN cell asin Example 1 to measure its threshold voltage. The results are shown inFIG. 9. Further, from the results in FIG. 9 was calculated thetemperature dependency of the threshold voltage (ΔV_(th) /Δt). Theresults are shown in Table 5.

For comparison, the results of Comparative example 1 are again shown inFIG. 9 and Table 5.

                  TABLE 5                                                         ______________________________________                                                                Comparative                                                           Example 3                                                                             example 1                                             ______________________________________                                        ΔV.sub.th /Δt                                                              -30˜25° C.                                                                    -1.1      -3.8                                                    25˜80° C.                                                                    -6.0      -8.7                                          ______________________________________                                    

In Example 3, it is seen that the temperature dependency of thethreshold voltage was considerably reduced.

EXAMPLE 4

To the nematic liquid crystal composition (D) (100 parts by weight)shown in Example 1 was added a compound B-6 (0.5 part by weight) as anoptically active substance, which compound is expressed by the sameformula as that of the compound B-5 shown in Example 3 but is obtainedusing S(+)-2-octanol as a starting raw material and has a left helicaltwist sense, to prepare a nematic liquid crystal composition, which wasthen sealed in the same cell as in Example 1 to measure its thresholdvoltage. The results are shown in FIG. 10. Further, from the results inFIG. 10 was calculated the temperature dependency of the thresholdvoltage (ΔV_(th) /Δt). The results are shown in Table 6. For comparison,the results of Comparative example 2 are again shown in FIG. 10 andTable 6.

                  TABLE 6                                                         ______________________________________                                                                Comparative                                                           Example 4                                                                             example 2                                             ______________________________________                                        ΔV.sub.th /Δt                                                              -30˜25° C.                                                                    +1.5      -4.7                                                    25˜80° C.                                                                    -4.5      -6.5                                          ______________________________________                                    

In the case of Example 4, there is shown a tendency that the temperaturedependency of the threshold voltage in the lower temperature region iscontrary to conventional one, as in the case of Example 2.

EXAMPLE 5 AND COMPARATIVE EXAMPLE 3

The following ten compounds of B-7 to B-16 as optically activesubstances belonging to the compounds respectively expressed by theformula (Ia), (Ib) or (Ic), and the above-mentioned six compounds of B-1to B-6, were respectively singly and in a quantity of 1 to 5 parts byweight added to the above nematic liquid crystal composition (A) (100parts by weight) referred to in the afore-mentioned description of theprior art, to prepare sixteen nematic liquid crystal compositions:##STR79##

As to these nematic liquid crystal compositions, their intrinsic helicalpitches were measured at various temperatures according to Cano wedgemethod, and the temperature dependency of the twistability calculatedfrom the results is shown in terms of normalized values of ΔP⁻¹ ₂₀˜40 inTable 7. For comparison, to the nematic liquid crystal composition (A)(100 parts by weight) were added the following six optically activesubstances C-5 to C-10 belonging to the group of the compounds expressedby the formula (II) and the above four optically active compounds C-1 toC-4, respectively singly and in a quantity of 0.5 to 5 parts by weight,to prepare ten nematic liquid crystal compositions, followed bymeasuring their intrinsic helical pitches in the same manner as inExample 5, to calculate their temperature dependencies of thetwistability. The resulting values of ΔP⁻¹ ₂₀˜40 are shown in Table 7.##STR80##

                  TABLE 7                                                         ______________________________________                                        Example 5                                                                     Compound            Comparative example 3                                     added   ΔP.sup.-1 20˜40                                                               Compound added                                                                             ΔP.sup.-1 20˜40                  ______________________________________                                        B-1     1.93        C-1          -0.48                                        2       0.92                                                                          -0.26                                                                 3       1.88                                                                          -0.28                                                                 4       1.93                                                                          -0.75                                                                 5       0.77                                                                          -0.63                                                                 6       0.77                                                                          -0.94                                                                 7       1.31                                                                          -0.68                                                                 8       1.35                                                                          -0.43                                                                 9       1.86                                                                          -0.13                                                                 10      1.09                                                                          -0.69                                                                 11      1.05                                                                  12      0.57                                                                  13      1.61                                                                  14      1.85                                                                  15      0.38                                                                  16      0.84                                                                  ______________________________________                                    

As described above, the composition in the first aspect of the presentinvention is characterized in that the value of ΔP⁻¹ representing thetemperature dependency of the twistability is a positive value, and asits effectiveness, it is possible to inhibit reduction in the thresholdvoltage of liquid crystal display elements using the composition of thepresent invention, accompanying the temperature rise.

EXAMPLE 6

To the above nematic liquid crystal composition (A) were added the aboveoptically active substances B-1 and C-3, in various mixing proportionsbut so as to give a total quantity of 1% by weight, to measure thetemperature dependency of the twistability (P⁻¹) of the resulting liquidcrystal composition. The fact that the optically active compounds B-1and C-3 had the same twist sense was confirmed according to contactmethod. Change in the ΔP⁻¹ ₂₀₋₇₀ in varied mixing proportions of B-1 andC-3 is shown in FIG. 11. When the quantity of B-1 added is 0.43% byweight, ΔP⁻¹ =0. This indicates that the intrinsic pitch P is unchangedbetween 20° C. and 70° C. at this mixing proportion.

COMPARATIVE EXAMPLE 4

The above optically active substance C-1 and the above optically activesubstance C-2 having an opposite twist sense to that of the abovesubstance C-1 were added in varied mixing proportions but so as to givea total quantity of 1% by weight, to the above nematic liquid crystalcomposition (A) to measure the temperature dependencies of thetwistability P⁻¹ of the resulting liquid crystal compositions. Resultsin the case where the quantity of C-1 added was varied from 0% by weightto 1% by weight at intervals of about 0.2% by weight are shown in FIG.12.

There is expected a temperature at which P⁻¹ =0 in the vicinity of thequantity of C-1 added of 0.6% by weight, and P⁻¹ takes a negative orpositive value below or above this temperature.

The change of ΔP⁻¹ between 20° C. and 70° C. in varied mixingproportions of C-1 and C-2 is shown in FIG. 13.

When the quantity of C-1 added is 0.46% by weight, ΔP⁻¹ =0, but when itis in the vicinity of 0.6% by weight, there is a temperature at whichP⁻¹ =0 and ΔP⁻¹ is divergent. This is because, as apparent from theequation (4), since the sign of P⁻¹ (20) and P⁻¹ (70) are different, theabsolute value of the ratio of the difference between P⁻¹ (20) and P⁻¹(70) to the sum of the two is necessarily 1 or more and |ΔP⁻¹ ₂₀₋₇₀ | isnecessarily 4 or more. Equation of P⁻¹ (20)=-P⁻¹ (70) comes intoexistence in a certain mixing ratio in which the denominator of theright side term of the equation (4) approaches zero; hence ΔP⁻¹ isdivergent.

In comparison of Example 6 with Comparative example 4, ΔP⁻¹ slowlychanges in the case of Example 6; whereas in the case of Comparativeexample 4, since two kinds of optically active substances having righttwist and left twist, respectively are mixed, there is a mixingproportion in which ΔP⁻¹ is divergent. Further, in comparison of theranges of mixing proportion within which P⁻¹ is almost unchanged (i.e.-0.1≦ΔP⁻¹ ≦0.1), the range in the case of Example 6 is as relativelybroad as 0.32˜0.53% by weight, whereas the range in the case ofComparative example 4 is 0.41˜0.49% by weight, that is, only half of therange in the case of Example 6. Since the range of mixing proportionwithin which ΔP⁻¹ ≈0, in the case of Example 6 is broader than that inthe case of Comparative example 4, it is seen that the intrinsic pitchis easily made constant irrespective of temperature.

In comparison of the ranges of mixing proportion within which P⁻¹increases with temperature rise (ΔP⁻¹ ≧0.1), the range in the case ofExample 6 is as very broad as 0.53˜1.0% by weight, whereas the range inthe case of Comparative example 4 is as very narrow as 0.5˜0.59% byweight, i.e. just before ΔP⁻¹ is divergent. Since the range of mixingproportion with which ΔP⁻¹ ≧0.1, in the case of Example 6, is muchbroader than that in the case of Comparative example 4, the temperaturedependency of the intrinsic pitch is more easily controlled in the caseof Example 6. Further, P⁻¹ at 20° C. in the case of ΔP⁻¹ =0 was roughlyestimated employing the above equation (3). The results are shown inTable 8.

                  TABLE 8                                                         ______________________________________                                                 Example 6                                                                             Comparative example 4                                        ______________________________________                                        P.sup.-1 (20)                                                                            0.036     0.028                                                    ______________________________________                                    

The value of P⁻¹ (20) in Example 6 is larger than that in Comparativeexample 4. Further, comparison of P⁻¹ s at 20° C. in the case where ΔP⁻¹is larger, is shown in Table 9.

                  TABLE 9                                                         ______________________________________                                                  Example 6                                                                             Comparative example 4                                       ______________________________________                                        ΔP.sup.-1 20-70                                                                     1.41      1.58                                                    P.sup.-1 (20)                                                                             0.0104    0.0044                                                  ______________________________________                                    

In this Table, close values of ΔP⁻¹ ₂₀₋₇₀ are chosen. The value ofP⁻¹.sub.(20) in the case of Comparative example 4 is less than half ofthat in the case of Example 6. This indicates that in order to obtain adefinite intrinsic pitch, as apparent from the equation (1), thequantity of optically active substances added in the case of Example 6may be sufficient to be less than half of that in the case ofComparative example 4. This fact that a small quantity of opticallyactive substances added may be sufficient affords two advantages thatthe resulting composition is cheap as much and the influence which theaddition has upon the characteristics of the original nematic liquidcrystal composition is small.

EXAMPLE 7

The above optically active substance B-4 included in the group ofcompounds expressed by the formula (III) and obtained usingS(+)-2-octanol as a starting raw material and having a left twist sense,and the above optically active substance C-9 included in the formula(VII), and obtained using S(+)-2-octanol as a starting raw material andhaving the same i.e. left twist sense, were added in varied mixingproportions but so as to give a total quantity of 1% by weight, to theabove nematic liquid crystal composition A, to measure the temperaturedependency of the twistability P⁻¹ of the resulting liquid crystalcomposition. The fact that the twist senses of the optically activesubstances B-4 and C-9 are the same was confirmed according to contactmethod. Change of ΔP⁻¹ ₂₀₋₇₀ in varied mixing proportions of B-4 and C-9is shown in FIG. 14. When the quantity of B-4 added is 0.28% by weight,ΔP⁻ =0. The mixing proportion of B-4 in which ΔP⁻¹ ≈0 (-0.1≦ΔP⁻¹ ≦0.1)is as very broad as 0.04˜0.46% by weight. Further, the mixing proportionof B-4 in which ΔP⁻¹ ≧0.1 is as very broad as 0.46˜1.0% by weight.

COMPARATIVE EXAMPLE 5

The above optically active substance C-3 and the above optically activesubstance C-6 having an opposite twist sense to that of the formersubstance were added to the above nematic liquid crystal composition (A)in varied mixing proportions but so as to give a total quantity of 1% byweight, to measure the temperature dependency of the twistability P⁻¹ ofthe resulting liquid crystal composition. The change of ΔP⁻¹ ₂₀₋₇₀ invaried mixing proportions of C-3 and C-6 is shown in FIG. 15. When thequantity of C-3 added is 0.86% by weight, ΔP⁻¹ =0. The range of thequantity thereof added in which ΔP⁻¹ ≈0 (-0.1≦ΔP⁻¹ ≦0.1) is as narrow as0.83˜0.89% by weight. Further, ΔP⁻¹ is also divergent. Further themixing proportion in which ΔP⁻¹ ≧0.1 has as narrow a range as 0.83˜0.7%by weight and is in the vicinity where ΔP⁻¹ is divergent.

The range of the mixing proportion in which ΔP⁻¹ ≈0 in the case ofExample 7 is much broader than that in the case of Comparative example5; hence the intrinsic pitch P is easily made constant irrespective oftemperature. Further, the range of the mixing proportion in which ΔP⁻¹≧0.1 in the case of Example 7 is much broader than that in the case ofComparative example 5; hence it is easy to control the temperaturedependency of the intrinsic pitch P in order to reduce the temperaturedependency of the threshold voltage V_(th). Further, P⁻¹ at 20° C. inthe case of ΔP⁻¹ =0 was roughly estimated. The results are shown inTable 10.

                  TABLE 10                                                        ______________________________________                                                 Example 7                                                                             Comparative example 5                                        ______________________________________                                        P.sup.-1 (20)                                                                            0.061     0.032                                                    ______________________________________                                    

The value of P⁻¹.sub.(20) in the case of Comparative example 5 is abouta half of that in the case of Example 7. Further, comparison of P⁻¹ s at20° C. in the case of larger ΔP⁻¹ is shown in Table 11.

                  TABLE 11                                                        ______________________________________                                                  Example 7                                                                             Comparative example 5                                       ______________________________________                                        ΔP.sup.-1 20-70                                                                     1.52      1.92                                                    P.sup.-1 (20)                                                                             0.0101    0.0056                                                  ______________________________________                                    

The P⁻¹.sub.(20) value in the case of Comparative example 5 is about ahalf of that in the case of Example 7. In short, the quantity ofoptically active substances added, in the case of Example 7, may besufficient to be a half of that in the case of Comparative example 5;hence Example 7 is superior in that the resulting composition is cheaperand the effect upon the characteristics of the original nematic liquidcrystal composition is small.

EXAMPLE 8

The above optically active substance B-4 belonging to the group ofcompounds expressed by the formula (IV); described in Japanese patentapplication No. Sho 60-283110/1985 (filed by Chisso Corporation);obtained using R(-)-2-octanol as a starting raw material; and having aright helical twist sense, and the above optically active substance C-3having the same twist sense as that of the above substance, were addedin varied mixing proportions but so as to give a total quantity of 1% byweight, to the above nematic liquid crystal composition (A), to measurethe temperature dependency of the twistability P⁻¹ of the resultingliquid crystal composition. The fact that B-14 and C-3 have the sametwist sense was confirmed according to contact method.

The change of ΔP⁻¹ ₂₀₋₇₀ in varied mixing proportions of B-14 and C-3 isshown in FIG. 16. When the quantity of B-14 added is 0.45% by weight,ΔP⁻¹ =0, and the mixing proportion of B-14 in which ΔP⁻¹ ≈0 (i.e.-0.1≦ΔP⁻¹ ≦0.1) has as very broad a range as 0.31-0.55% by weight. Themixing proportion in which ΔP⁻¹ ≧0.1 has as broad a range as 0.55-1.0%by weight.

In comparison of Example 8 with Comparative example 5, the range of themixing proportion in which ΔP⁻¹ ≈0 in the case of Example 8 is muchbroader than that in the case of Comparative example 5; hence theintrinsic pitch P is easily made constant irrespective of temperature.Further, the range of the mixing proportion in which ΔP⁻¹ ≧0.1, in thecase of Example 8, is much broader than that in the case of Comparativeexample 5; hence it is easy to control the temperature dependency of theintrinsic pitch P in order to reduce the temperature dependency of thethreshold voltage V_(th).

Further, P⁻¹ at 20° C. in the case of ΔP⁻¹ =0 was roughly estimated. Theresults are shown in Table 12.

                  TABLE 12                                                        ______________________________________                                                 Example 8                                                                             Comparative example 5                                        ______________________________________                                        P.sup.-1 (20)                                                                            0.037     0.032                                                    ______________________________________                                    

The value of P⁻¹.sub.(20) in the case of Example 8 is larger than thatin the case of Comparative example 5.

Further, comparison of P⁻¹ s at 20° C. in which ΔP⁻¹ is large is shownin Table 13.

                  TABLE 13                                                        ______________________________________                                                  Example 8                                                                             Comparative example 5                                       ______________________________________                                        ΔP.sup.-1 20-70                                                                     1.21      1.92                                                    P.sup.-1 (20)                                                                             0.0094    0.0056                                                  ______________________________________                                    

The value of P⁻¹.sub.(20) in the case of Example 8 is about twice thevalue in the case of Comparative example 5. In short, the quantity ofoptically active substance added in the case of Example 8 may besufficient to be less than that in the case of Comparative example 5;hence Example 8 is superior in that the resulting composition is cheaperand also the effect upon the characteristics of the original nematicliquid crystal composition is small.

Examples 6, 7 and 8 indicate that when an optically active substancehaving a positive and very large ΔP⁻¹ ₂₀₋₇₀, as expressed by the formula(III) or (IV), and an optically active substance having the same twistsense as that of the former substance and having a negative andrelatively small ΔP⁻¹ ₂₀₋₇₀ are combined, then it is possible to easilycontrol the temperature dependency of the intrinsic pitch P in order tobe free from the temperature dependency of the twistability P⁻¹ or toreduce the temperature dependency of the threshold voltage V_(th). It isseen from these facts that in order to control the temperaturedependency of the intrinsic pitch P to thereby reduce the temperaturedependency of the threshold voltage V_(th), the mixture of an opticallyactive substance having a positive and large ΔP⁻¹ value and an opticallyactive substance having the same twist sense as that of the abovesubstance and a negative value of ΔP⁻¹ broadens range of mixingproportion in which ΔP⁻¹ ≧0. Further, when the pitch of eithercomponents is shorter at room temperature, a smaller addition quantitythereof may be sufficient.

EXAMPLES 9˜11 AND COMPARATIVE EXAMPLE 6

An optically active substance B-17 belonging to a group of compounds ofthe formula (III) and expressed by the formula ##STR81## which substanceis disclosed in Japanese patent application laid-open No. Sho 61-43;obtained using S(+)-2-octanol as a starting raw material; and has a lefthelical twist sense, and an optically active substance C-11 expressed bythe formula ##STR82## which substance has the same twist sense as thatof the above substance; is disclosed in Japanese patent applicationlaid-open No. Sho 60-149548; and is obtained using S(+)-2-octanol as astarting raw material, were added to the above nematic liquid crystalcomposition D in varied mixing proportions so as to give an intrinsicpitch at 25° C. of 80 μm, to prepare 4 kinds of nematic liquid crystalcompositions shown in Table 14.

Table 14 shows the mixing proportions of B-17 and C-11 in the respectivecompositions, the total quantities of the two compounds added and theΔP⁻¹ ₂₀₋₄₀. The fact that the twist senses of the optically activesubstances B-17 and C-11 are the same was confirmed according to contactmethod.

                  TABLE 14                                                        ______________________________________                                               Mixing propor-                                                                           Total quantity                                                     tion of B-17                                                                             added       ΔP.sup.-1 20-40                           ______________________________________                                        Example 9                                                                               2.4 wt. %    2.4 wt. %  1.79                                        Example 10                                                                             0.75 wt. %   0.83 wt. %  1.21                                        Example 11                                                                             0.40 wt. %   0.50 wt. %  0.55                                        Comparative                                                                              0 wt. %    0.12 wt. %  -0.03                                       example 6                                                                     ______________________________________                                    

These 4 kinds of the compositions were sealed in the same TN cell as inExample 1 to measure their threshold voltages. The results are shown inFIG. 17, and from the results were calculated their temperaturedependencies of the threshold voltages, which are shown in Table 15.

                  TABLE 15                                                        ______________________________________                                                    ΔV.sub.th /Δt                                                     -30˜25° C.                                                                25˜80° C.                                   ______________________________________                                        Example 9     +10.7      -3.5                                                 Example 10    0          -6.7                                                 Example 11    -1.8       -7.3                                                 Comparative   -4.2       -7.8                                                 example 6                                                                     ______________________________________                                    

It is seen from FIG. 17 and Table 15 that in the case of Example 9, thetemperature dependency of the threshold voltage is contrary to usual oneon the lower temperature side, while the absolute value of the ΔV_(th)/Δt is very small on the higher temperature side. In the cases ofExamples 10 and 11, the absolute values of the ΔV_(th) /Δt are farsmaller than that in the case of Comparative example 6 on the lowertemperature side although there are no large difference therebetween onthe higher temperature side. Particularly in the case of Example 10,they are constant irrespective of temperature on the lower temperatureside.

As described above, by combining an optically active substance of theformula (Ia) with an optically active substance of the formula (II) bothhaving the same twist sense, it is possible to control the temperaturedependency of the twistability and also to reduce the temperaturedependency of the threshold voltage.

EXAMPLE 12

The above optically active substance B-12 belonging to the group ofcompounds of the formula (III) and disclosed in the Japanese patentapplication laid-open No. Sho 61-43, which substance is obtained usingS(+)-2-pentanol as a starting raw material and has a left twist sense,and the optically active substance C-9 having the same twist sense asthat of the former substance were added in varied mixing proportions butso as to give a total quantity of 1% by weight to the nematic liquidcrystal composition (A), to measure the temperature dependency of thetwistability P⁻¹.

The change of ΔP⁻¹ ₂₀₋₇₀ in varied mixing proportions of B-12 and C-9 isshown in FIG. 18. When the quantity of B-12 added is 0.57% by weight,ΔP⁻¹ =0, and the range of the quantities thereof added in which ΔP⁻¹ ≈0(-0.1≦ΔP⁻¹ ≦0.1) is as very broad as 0.14˜0.74% by weight.

In comparison of Example 12 with Comparative examples 4 and 5, the rangeof the mixing proportions in which ΔP⁻¹ ≈0 is also very broad in Example12. According to the prior art, optically active substances having thetwist senses which are opposite to one another are mixed together sothat the range in which ΔP⁻¹ ≈0 is in the vicinity where ΔP⁻¹ isdivergent, whereby the range of the mixing proportion in which ΔP⁻¹ ≈0is necessarily very narrow, and in order to make the intrinsic pitchconstant irrespective of temperature, the very delicate mixingproportion should be exactly determined. According to the presentinvention, however, by mixing and adding optically active substancesboth having the same twist sense, it is possible to prepare a nematicliquid crystal composition of ΔP⁻¹ ≈0; hence it has become very easy tocontrol the temperature dependency of the intrinsic pitch.

Further, in comparison of the value of the twistability at 20° C. in thecase of ΔP⁻¹ ≈0 in Example 12 with that in Comparative example 4, theresults shown in Table 16 are obtained.

                  TABLE 16                                                        ______________________________________                                                  Example 12                                                                            Comparative example 4                                       ______________________________________                                        ΔP.sup.-1 20-70                                                                     0.01      0.14                                                    P.sup.-1 (20)                                                                             0.040     0.021                                                   ______________________________________                                    

The value of P⁻¹.sub.(20) in the case of Comparative example 4 is abouta half of that in the case of Example 12. In short, in the case ofExample 12, a smaller quantity of optically active substances may besufficient as compared with that in the case of Comparative example 4.

EXAMPLE 13

The above optically active substance B-15 belonging to the group of thecompounds of the formula (IV) and disclosed in Japanese patentapplication No. Sho 60-51512/1985 filed by Chisso Corporation, whichsubstance is obtained using S(+)-2-octanol as a starting raw materialand has a left twist sense, and the above optically active substance C-9having the same twist sense as that of the former substance, were addedin varied mixing proportions but so as to give a total quantity of 1% byweight, to the above nematic liquid crystal composition (A) to measurethe temperature dependency of the twistability P⁻¹ of the resultingliquid crystal composition. The change of ΔP⁻¹ ₂₀₋₇₀ in varied mixingproportions of the compounds B-15 and C-9 is shown in FIG. 19. When thequantity of B-15 added is 0.63% by weight. ΔP⁻¹ =0, and the range of thequantities thereof added in which ΔP⁻¹ ≈0 (i.e. -0.1≦ΔP⁻¹ ≦0.1) is asvery broad as 0.1˜0.88% by weight.

In the case of Example 13, the range of the mixing proportions in whichΔP⁻¹ ≈0 is much broader than those in the cases of Comparative examples4 and 5; hence it is very easy to control the temperature dependency ofthe intrinsic pitch.

Further, the value of the twistability P⁻¹ at 20° C. in the mixingproportion in which ΔP⁻¹ ≈0 was compared with that in Comparativeexample 4. The results are shown in Table 17.

                  TABLE 17                                                        ______________________________________                                                  Example 13                                                                            Comparative example 4                                       ______________________________________                                        ΔP.sup.-1 20-70                                                                     0.01      0.14                                                    P.sup.-1 (20)                                                                             0.046     0.021                                                   ______________________________________                                    

The value of P⁻¹.sub.(20) in Example 13 is twice or more the value inComparative example 4. In short, in the case of Example 13, a smallerquantity of the optically active substance may be sufficient as comparedwith that in Comparative example 4.

EXAMPLE 14

The above optically active substance B-16 belonging to the formula (V),obtained using R(-)-2-octanol as a starting raw material and having aright twist sense, and an optically active substance C-12 having thesame formula as that of the former substance C-9 but having a righttwist substance, were added in varied mixing proportions but so as togive a total quantity of 1% by weight to the above nematic liquidcrystal composition (A), to measure the temperature dependency of thetwistability P⁻¹ of the resulting liquid crystal composition. The valuesof ΔP⁻¹ ₂₀₋₇₀ in varied mixing proportions are shown in FIG. 20. Whenthe quantity of B-16 added is 0.22% by weight, ΔP⁻¹ =0, and the range ofthe quantities thereof added in which ΔP⁻¹ ≈0 (-0.1≦ΔP⁻¹ ≦0.1) is asbroad as 0.0˜0.41% by weight.

In this Example, the range of the mixing proportions in which P⁻¹ ≈0 ismuch broader than that in Comparative examples 4 and 5; hence it is veryeasy to control the temperature dependency of the intrinsic pitch.Further the value of P⁻¹ at 20° C. in the case of ΔP⁻¹ =0 was roughlyestimated employing the above equation (3). The results are shown inTable 18.

                  TABLE 18                                                        ______________________________________                                                 Example 14                                                                            Comparative example 4                                        ______________________________________                                        P.sup.-1 (20)                                                                            0.048     0.028                                                    ______________________________________                                    

The value of P⁻¹.sub.(20) in Example 14 is about twice the value inComparative example 4. This indicates that as apparent from the equation(1), in order to obtain a definite intrinsic pitch, the quantity ofoptically active substances added in Example 14 may be sufficient to beabout a half of that in Comparative example 4. The fact that a smallquantity of optically active substances added may be sufficient affordstwo advantages that the resulting composition is cheap as much and thereis no influence upon the characteristics of the original liquid crystalcomposition.

EXAMPLE 15

The above optically active substances B-5 and C-3 were added in variedmixing proportions but so as to give a total quantity of 1% by weight tothe above nematic liquid crystal composition (A) to measure thetemperature dependency of the twistability P⁻¹ of the resulting liquidcrystal composition. The values of ΔP⁻¹ ₂₀₋₇₀ in varied mixingproportions are shown in FIG. 21. When the quantity of B-5 added is0.40% by weight, ΔP⁻¹ =0, and the range of the mixing proportions inwhich ΔP⁻¹ ≈0 (-0.1≦ΔP⁻¹ ≦0.1) is as broad as 0.30˜0.55% by weight.Further, the range of the mixing proportions in which ΔP⁻¹ ≧0.1 is asvery broad as 0.55-1.0% by weight.

In comparison of Example 15 with Comparative example 5, the range of themixing proportion in which ΔP⁻¹ ≈0 in the case of Example 15 is broaderthan that in the case of Comparative example 5; hence it is easy to makethe intrinsic pitch P constant irrespective of temperature. Further,since the range of the mixing proportions in which ΔP⁻¹ ≧0.1 in the caseof Example 15 is broader than that in the case of Comparative example 5,it is easy to control the temperature dependency of the intrinsic pitchP to thereby reduce the temperature dependency of the threshold voltageV_(th). Further, the values of P⁻¹ at 20° C. in the case of P⁻¹ =0 wereroughly estimated. The results are shown in Table 19.

                  TABLE 19                                                        ______________________________________                                                 Example 15                                                                            Comparative example 5                                        ______________________________________                                        P.sup.-1 (20)                                                                            0.048     0.032                                                    ______________________________________                                    

The value of P⁻¹.sub.(20) in Comparative example 5 is considerablysmaller than that in Example 15. In short, since the quantity ofoptically active substance added in Example 15 may be sufficient to besmaller than that in Comparative example 5, Example 15 is superior inthat the resulting composition is cheap and the influence upon thecharacteristics of the original nematic liquid crystal composition issmall.

When the composition is applied to SBE mode, it is necessary to make thevalue of the intrinsic pitch P about 10 μm, taking a usual cellthickness of about 7 μm into account. In order to make the P value 10μm, the total addition quantity is 2.5% by weight in the case of Example12; 2.2% by weight in the case of Example 13; 2.1% by weight in the caseof Example 14; and 2.1% by weight in the case of Example 15. On theother hand, in order to make the P value 10 μm, the total additionquantity is 4.8% by weight in the case of Comparative example 4.

Since the optically active substance C-1 of Comparative example 4 is inthe form of a transparent liquid at room temperature, the NI point ofthe resulting liquid crystal composition considerably lowers due toconsiderable increase in the quantity thereof added. In order to preventthe composition from lowering the upper limit temperature of the nematicrange, it is necessary to add another liquid crystal compound having ahigh clearing point (a high temperature liquid crystal compound).

Since high temperature liquid crystal compounds generally have a highvisocisity, the viscosity of the resulting liquid crystal compositionrises, and further since the optically active substances themselves havea high viscosity, the viscosity of the liquid crystal composition risesconsiderably so that such a viscosity rise brings about a drawback ofreducing the response speed.

In the case of the above TN mode, an intrinsic pitch of about 100-200 μmis generally used, and so a small addition quantity may be sufficient;hence the effect upon the N-I point or the viscosity is small. Whereas,in the case of SBE mode or the like wherein a shorter pitch is required,the addition quantity increases to a large extent. Nevertheless, it isdesired to inhibit bad influence due to addition of optically activesubstances to the utmost.

Examples 12-15 are directed to the case where an optically activecompound as expressed by the formula (III) or (V), and having a ΔP⁻¹₂₀₋₇₀ value which is positive but not so large, is combined with anoptically active substance having the same twist sense as that of theformer substance, and also having a ΔP⁻¹ ₂₀₋₇₀ value which is negativevalue and relatively small.

From the results of these Examples it is seen that for the purpose ofbeing free from the temperature dependency of the intrinsic pitch P inorder to use the compositions for SBE mode, DGH mode or PC mode,addition of an optically active substance having a positive and smallΔP⁻¹ value and an optically active substance having the same twist senseas that of the former substance and also having a negative and smallΔP⁻¹ value, broadens the range of the mixing proportion in which ΔP⁻¹≈0; hence the object is very easily attained. Further, the opticallyactive components both are preferred to have a shorter pitch at roomtemperature, because a smaller addition quantity is required.

As described above, it is commercially very important that addition ofonly a small quantity of optically active substances makes it possibleto very easily control the intrinsic pitch of nematic liquid crystalcompositions.

What we claim is:
 1. A nematic liquid crystal composition comprising:(i) an optically active component which makes positive the temperaturedependency of the twistability in terms of the reciprocal of theintrinsic helical pitch thereof, of the cholesteric phase induced whensingly added to a nematic liquid crystal and which consists of oneoptically active substance or at least two optically active substanceshaving the same helical twist sense to one another, said opticallyactive substances being selected from the group consisting of compoundsexpressed by the formula (III), compounds expressed by the formula (IV),compounds expressed by the formula (V), a compound expressed by theformula (B-7) and a compound expressed by the formula (B-11), ##STR83##wherein, in the formula (III), R represents an alkyl group or an alkoxygroup each of 1 to 15 carbon atoms or a cyano group;R¹ represents alinear chain alkyl group of 2 to 10 carbon atoms; a represents aninteger of 0, 1 or 2; b represents an integer of 1 or 2; the value of(a+b) is 2 or 3; V₁ represents a single bond when a=0, and when a is 1or 2, V₁ represents --COO--, --OCO--, --CH₂ O--, --OCH₂ -- or --CH₂ CH₂--; in the formula (IV), R¹ represents a linear chain alkyl group of 2to 10 carbon atoms; R² represents an alkyl group or an alkoxy group eachof 1 to 15 carbon atoms, a cyano group or a halogen atom; T¹, T², T³ andT⁴ each independently represents a hydrogen atom, a halogen atom or acyano group; c and d each represent an integer of 0 or 1; and V₂represents a single bond, --COO--, or --OCO--; and in the formula (V),R¹ and R³ each independently represent a linear chain alkyl group of 2to 10 carbon atoms; e represents an integer of 0, 1 or 2; f representsan integer of 1 or 2; the value of (e+f) is 1 to 3; V₃ represents asingle bond when e=0, and when e is 1 or 2, V₃ represents --COO-- or--CH₂ O--, and (ii) at least one nematic liquid crystal compound, saidoptically active component being contained in a quantity in the range of0.05 to 10% by weight in said composition.
 2. A nematic liquid crystalcomposition according to claim 1 wherein said optically activesubstances are expressed by the formula (III) in claim
 1. 3. A nematicliquid crystal composition according to claim 1 wherein said opticallyactive substances are expressed by the formula (IV) in claim
 1. 4. Anematic liquid crystal composition according to claim 1 wherein saidoptically active substances are expressed by the formula (V) in claim 1.5. A liquid crystal display element comprising a liquid crystalcomposition as set forth in claim
 1. 6. A liquid crystal display elementcomprising a liquid crystal composition as set forth in claim
 2. 7. Aliquid crystal display element comprising a liquid crystal compositionas set forth in claim
 3. 8. A liquid crystal display element comprisinga liquid crystal composition as set forth in claim 4.